In recent years, the density of integrated circuits, particularly those of microprocessors, has increased greatly enabling relatively massive quantities of data to be processed within a single integrated circuit. This has led to a commensurate increase in the amount and speed of data transferred externally of the integrated circuit together with increase in the number and density of integrated circuit pins enabling such data transfer, in particular, enabling the parallel transmission of large quantities of data. The consequential extreme miniaturization and very high clock frequencies of central processing units, thinness and close spacing of pins has increased the possibility of cross-talk between pins and, with the increasing switching speeds, the current flow from such high speed switching has dramatically increased in the high frequency range. As a result, in current supply circuits, variations in the source voltage can easily occur producing false switching signals in the electronic circuits. Furthermore, the increased frequency of the signals has increased the difficulty of controlling the impedance between the pins increasing the risk of impedance mismatch with other circuits connected thereto.
EP-A- 86 961 discloses a multilayer board for the interconnection of high-speed circuits in which a first voltage layer is sandwiched between two ground layers at a close spacing to produce a large distributed capacitance inhibiting switching signals from causing voltage spikes in the power lines and reducing the inductance of the conductive layers thereby improving the characteristic impedance of the board.
However, this prior proposal suffers from disadvantages concerning, for example, effective shielding of board mounted components and cost of manufacture.
Other multilayer circuit boards are known, for example from U.S. Pat. No. 4,489,999, which teaches a flexible board assembly integral with an interconnect socket and U.S. Pat. No. 4,208,080, which teaches a stack of terminal board sandwiched between rigid housing parts for current distribution in an automobile.